1. Field of the Invention
The present invention relates to an electronic musical instrument of the type that generates a musical waveform by computing the amplitude value of a musical waveform at each sample point thereof through Fourier synthesis, and more particularly to an electronic musical instrument which is adapted so that harmonic coefficient for setting a timbre is varied with time and in accordance with a touch response.
2. Description of the Prior Art
Heretofore, there have been proposed many digital type electronic musical instruments which produce the amplitude value of a musical waveform at each sample point thereof by some method and read it out at a readout rate corresponding to a note frequency. The simplest one of them is what is called a "waveform-memory method" which stores and reads out waveform data itself, and a method that converts an analog input to digital form to obtain waveform data is also one of the simplest method. However, these conventional methods are defective in that an enormous memory capacity is needed for variations in the musical waveform in accordance with particular sound ranges and in that the musical waveform undergoes no temporal variations. Furthermore, there have also been considered a method of computing paramters through the use of various continous functions and a method of computing temporal variations in the musical waveform in a real-time waveform synthesis by a frequency modulation method, but the correspondence between a parameter for the waveform generation and the timbre of the musical sound actually produced is unnatural to the human sense, and a desired timbre is difficult to obtain.
On the other hand, a musical waveform generating system utilizing Fourier synthesis has undergone various improvements to make up for the defect of a large volume of waveform synthesis calculation and has been widely employed since parameters for harmonic coefficients naturally correspond to an auditory evaluation of timbre. In the musical waveform generation system utilizing Fourier synthesis, it is the component ratio of a harmonic coefficient that determines the timbre of a musical sound. As a method for causing temporal variations in the musical waveform, there has been suggested a method of selecting many harmonic coefficients by using a plurality of memories, but this method has such a shortcoming that sufficient timbre variations cannot be obtained in spite of an enormous circuit scale. Furthermore, a system which multiplies a preset harmonic coefficient and a parameter of a Formant filter, as described in Japanes Patent Publication No. 46445/78 and a system which multiplies a temporal variation function for each harmonic coefficient, as described in Japanese Patent Public Disclosure No. 172396/84, both require a multiplication circuit and possess such a defect that its circuit scale and operation time impose limitations on the entire system, resulting in temporal variations of the musical waveform being insufficient.